1. Field of the Invention
In general, the present invention relates to a chrominance signal processing circuit, a chrominance signal processing method and a camera using the circuit and the method. More particularly, the present relates to a chrominance signal processing circuit having an auto white balance function for automatically establishing white balance through feedback control, a chrominance signal processing method adopted in the circuit and a camera or an image sensing system using solid state image sensing devices such as image sensing devices of a CCD (Charge Coupled Device) type which employs the circuit.
2. Description of the Related Art
Establishment of white balance is adjustment of a white color seen as colored white to colorless white. The white balance adjustment is carried out typically in an image sensing system employing solid state image sensing devices when the white color moves along a black body radiation curve (a black body locus) in accordance with a change in color temperature as shown in FIG. 9, that is, when the color temperature of a photographic object serving as a light source changes. Normally, colored white appears as a reddish white color at a low color temperature and appears as a rather blue white color at a high color temperature. Here, by a color temperature, the temperature (expressed in terms of degrees Kelvin) of a black body having the same chromaticity as a test light source is meant. In FIG. 9, the origin represents the colorless white.
In the white balance adjustment, however, a color which is naturally not white has to be kept as it is. In order to prevent the white balance adjustment from being incorrectly applied to a color which is naturally not white, an lead-in limiting frame shown by a dotted line in FIG. 9 is provided. The lead-in limiting frame is used for limiting a range in which white balance adjustment is to be carried out. A color outside the lead-in limiting frame is not regarded as a drifted white color but is seen as a color which is naturally not white. For such a color, white balance adjustment is not carried out. That is, a lead-in operation is not performed.
In the conventional chrominance signal processing circuit having an auto white balance function adopting a feedback control system, however, once white balance has been established at a certain color temperature, the lead-in limiting frame moves in a manner interlocked with the establishment of the white balance. As a result, there is raised a problem that, when the color temperature changes next time, a color inside the initial lead-in limiting frame which is supposed to be subjected to white balance adjustment is displaced to a position outside the current lead-in limiting frame after the movement, that is, a position exempted from white balance. On the other hand, a color outside the initial lead-in limiting frame which should be exempted from white balance adjustment is displaced to a position inside the current lead-in limiting frame after the movement, that is, a position subjected to white balance adjustment.
As an example, assume a transition from a certain reference color temperature in a state where white balance has been established to a high color temperature. The transition causes the white color to be shifted in the blue color direction as shown in FIG. 10A. The white color shifted in the blue color direction is adjusted to colorless white shown in FIG. 10B. Then, in a next operation to establish white balance, the lead-in limiting frame shown by a solid line in FIG. 10B is used as a reference. For example, when a transition back to a low color temperature occurs as shown in FIG. 10C afterward, a color which was inside the original lead-in limiting frame shown by a dashed line is now located at a position outside the current lead-in limiting frame shown by the solid line, a position exempted from white balance adjustment.
In addition, there is also raised a problem that, in the case of a gradual change in color temperature, new colors are included in the lead-in limiting frame shifted to a new position, making it necessary to eventually apply white balance adjustment to a color at the end of the black body radiation curve representing an extreme color temperature even if the end color was originally outside the initial lead-in limiting frame prior to the shift. For example, assume that a transition has occurred from a state completing white balance adjustment at a reference temperature to a high color temperature where a white color is shifted in the blue color direction as shown in FIG. 11A. In this case, the white color shifted in the blue color direction is subjected to white balance adjustment to shift the white color to colorless white as shown in FIG. 11B. Thereafter, assume that a transition further occurs from the state shown in FIG. 11B to an even higher color temperature shown in FIG. 11C. In this case, a color at the end of the black body radiation curve which was originally outside the initial lead-in limiting frame shown by a dashed line in FIG. 11C eventually becomes subjected to white balance adjustment because the point is now located inside the current lead-in limiting frame indicated by a solid line.
In order to solve the problem described above, the patent applicant has proposed to change the size of the lead-in limiting frame in accordance with the color temperature as disclosed in Japanese Patent Laid-open No. Hei8-122903. To put it in detail, according to this disclosed method, in automatic white balance adjustment carried out by execution of feedback control, a change in color temperature of a light source is recognized. Then, the size of the lead-in limiting frame is changed in accordance with the color temperature of the light source at convergence upon white balance adjustment and the color temperature of the light source after a change in color temperature as shown in FIGS. 12A to 12B to result in control which continuously includes colors subjected to white balance adjustment in a reference lead-in limiting frame set from the beginning. In this method, however, if steps of changing the size of the lead-in limiting frame are not made fine, adequate control can not be executed so that it is feared that an incorrect operation will be resulted in. If the steps are made fine, however, the size of the program will become extremely large.
The present invention addresses the problem described above. It is thus an object of the present invention to provide a chrominance signal processing circuit and a chrominance signal processing method which are capable of carrying out processing with no white balance error as well as to provide a camera employing the circuit and adopting the method.
The chrominance signal processing circuit provided by the present invention comprises:
white balance amplifiers with relative gains among R, G and B primary color signals thereof adjusted by using a feedback control technique;
an integration circuit for generating integrated values of chrominance difference signals for each field; and
a controller for setting a lead-in limiting frame for controlling a range for establishing white balance with a reference point at a predetermined color temperature taken as a base and for continuously monitoring the magnitude of a shift of each of said integrated values output by said integration circuit from said reference point and, for integrated values existing inside said lead-in limiting frame after said shift, controlling said relative gains of said white balance amplifiers in accordance with said integrated values.
According to the chrominance signal processing method provided by the present invention, processing to automatically establish white balance by execution of feedback control comprises the steps of:
setting a lead-in limiting frame for controlling a range for establishing white balance with a reference point at a predetermined color temperature taken as a base; and
continuously monitoring the magnitude of a shift of each of said integrated values output by said integration circuit from said reference point and, for integrated values existing inside said lead-in limiting frame after said shift, controlling said gains of said white balance amplifiers in accordance with said integrated values.
The camera provided by the present invention employs the chrominance signal processing circuit with a configuration described above and adopts the chrominance signal processing method.
In white balance adjustment based on feedback control, first of all, at a certain determined color temperature, a reference point to be used as an operation start point is taken and then a lead-in limiting frame for not applying white balance adjustment on colors other than those on a black body radiation curve is set with the reference point taken as a basis. Subsequently, if the color temperature of a photographic object changes, a white balance lead-in operation based on integrated values which are output by the integrated circuit and exist inside the lead-in limiting frame is carried out. In this case, the lead-in limiting frame is not shifted even if the white balance lead-in operation is carried out.
Next, when the color temperature of the photographic object changes, the position or the coordinates of the current movement destination are determined by adding the current integrated values to the movement position or the coordinates of the previous integration values or subtracting the current integrated values from the movement position or the coordinates of the previous integration values depending on the movement direction. That is, the distances of the integrated values from the reference point are being monitored continuously. Then, the integrated values are examined to determine whether or not the values are in the lead-in limiting frame. If the integrated values are in the lead-in limiting frame, a while balance lead-in operation based on the integrated values is carried out. If the integrated values are not in the lead-in limiting frame, on the other hand, the previous position or coordinates are kept as they are till the color temperature of the photographic object changes.